We make the first report of Microsporidia MB in An. gambiae s.s and An. coluzzii following identification of the symbiont in An. arabiensis. This does not only demonstrate the existence of the microsporidian in another predominant malaria vector species in Africa but also extends its incidence from East to West Africa. The prevalence of MB-positive mosquitoes was estimated to be 1.8%, which is within the rate of < 1–9% reported for An. arabiensis2. The present study took advantage of archived mosquito DNA samples which were either collected from the field as larvae or adults from different study sites over 5 years.
Anopheles gambiae s.s and An. coluzzii are the predominant malaria vectors in Ghana7,8,9. They are often found in sympatry with one species usually being more abundant9,10. Contrary to the study by Herren and colleagues2, a handful of An. arabiensis was analysed in the present study. Anopheles arabiensis is more commonly found in the arid north of Ghana where rainfall is observed within a few months in a year. In studies conducted in Ghana that are focused on Anopheles distribution, between 2–3% are An. arabiensis despite the collection of large numbers of mosquitoes7,11. However, we acknowledge that there were more collections from the south of the country, especially Greater Accra, which contributed 40% of the DNA samples used in this study and 89% of MB-positive mosquitoes. Further studies to investigate variations in mosquito species density and seasonal prevalence of Microsporidia MB will shed more light on the field dynamics of the symbiont in these mosquito populations.
DNA samples from mosquitoes initially collected as larvae from the field showed significantly higher MB-positivity than those collected as adults. Microsporidians can be transmitted both vertically and horizontally12. The efficiency with which MB is transmitted vertically depends on the intensity in the ovaries of the female parent. Horizontal transfer was initially speculated to occur in the larval habitat2 which would increase the spread of MB from few infected larvae to many in the breeding site, but this transmission route has recently been shown not to occur in larvae under laboratory conditions13. In effect, our detection of higher prevalence among newly emerged adult mosquitoes is more likely to be attributed to highly efficient vertical transmission from female parent to offspring2. In the laboratory, the intensity of MB in adult mosquitoes has been observed to increase with age2. However, in the wild where conditions are very dynamic, the effect of larval habitat conditions and adult age on MB intensity in mosquitoes may differ. For example, larval diet has been shown to affect the microbial composition of mosquitoes in their adult life stages14. It is also established that microbial diversity decreases in female adults mainly because of proliferation of certain bacteria species following sugar and/or blood feeding15. It, therefore, makes it challenging to explain variations in bacterial diversity among field-caught adult mosquitoes since their feeding histories are unknown16. Several environmental and physiological statuses could potentially affect the intensity of MB observed in adults and variations in infection prevalence could be governed by factors that are yet to be investigated. Given that collecting large numbers of adult mosquitoes in the field may prove more challenging than larval sampling, our results have also shown the increasing chances of finding MB infections in a population when larvae are collected in field studies2.
While the data presented here shows basic information about the ecological spread of Microsporidia MB, it has nonetheless demonstrated a potential widespread occurrence of Microsporidia MB among Anopheles mosquitoes across sub-Saharan Africa. It warrants further investigation of the diversity, environmental dynamics, and interactions with other mosquito symbionts for a clearer understanding of their possible use in malaria control.
Source: Ecology - nature.com
